An antenna sharing device generally has two types of filters—a transmit filter and a receive filter—for separating signals of the transmitting passband from signals of the receiving passband that is adjacent to the higher side of the transmitting passband. The transmit filter is generally formed of a ladder type filter where series resonators and parallel resonators are connected in a ladder structure.
In Band 2 of the W-CDMA standard, the band spacing between the transmitting passband and the receiving passband (i.e. the crossband) is determined to be 20 MHz. In an antenna sharing device that copes with such a narrow crossband above, steepness in attenuation characteristics in the crossband between the transmitting passband and the receiving passband is significantly effective in ensuring isolation of transmitting signals and receiving signals. Specifically, what is needed is steepness in attenuation characteristics in frequencies higher than the passband of the transmit filter and steepness in attenuation characteristics in frequencies lower than the passband of the receive filter.
To improve the steepness in attenuation characteristics in frequencies higher than the passband of the transmit filter, a suggestion has introduced a technique in which weighed IDT electrodes are employed for the resonator of the transmit filter.
According to the conventional antenna sharing device, however, increase in steepness in attenuation characteristics has brought the following problems. In the transmit filter, increasing the steepness in attenuation characteristics in frequencies higher than the passband increases loss in the transmitting passband. Similarly, in the receive filter, increasing the steepness in attenuation characteristics in frequencies lower than the passband increases loss in the receiving passband.
As described above, the pending problem of the conventional antenna sharing device is to satisfy both of properly steep attenuation characteristics in a crossband and low loss in a passband.
As a related art patent literature preceding the present invention, patent literature 1 is known, for example.